build: add global soft timeout of 12 hours

Only start new builds for 12 hours. This ensures we publish new builds
often enough even on long backlogs.

This could be made configurable at a later point.

fdroidserver/build.py

@@ -1151,11 +1151,17 @@ def main():
     # Build applications...
     failed_apps = {}
     build_succeeded = []
+    # Only build for 12 hours, then stop gracefully
+    endtime = time.time() + 12 * 60 * 60
+    max_build_time_reached = False
     for appid, app in apps.items():
 
         first = True
 
         for build in app.builds:
+            if time.time() > endtime:
+                max_build_time_reached = True
+                break
             if options.server:  # enable watchdog timer
                 timer = threading.Timer(7200, force_halt_build)
                 timer.start()
@@ -1305,6 +1311,10 @@ def main():
             if timer:
                 timer.cancel()  # kill the watchdog timer
 
+        if max_build_time_reached:
+            logging.info("Stopping after global build timeout...")
+            break
+
     for app in build_succeeded:
         logging.info("success: %s" % (app.id))